Luminescent solar concentrators (LSCs) use down-converting phosphors embedded in a transparent waveguide to absorb sunlight, trap luminescent photons by total internal reflection, and deliver high irradiance, narrowband output light for driving photovoltaic, photochemical, and other solar energy converters. Quantum-dot-based (QD) LSCs are typically affected by several optical loss mechanisms including the loss of a fraction of guided light during transport to the concentrator edges through scattering from QD aggregates. Although the recent introduction of large effective Stokes shift QD luminophores for LSC applications has helped address several shortcomings associated with previous generations of organic and inorganic dyes, including improved solar spectrum matching, photostability, and photoluminescence quantum yield, achieving low light scattering at technologically relevant QD loadings in commercially deployed polymers such as poly(methyl methacrylate) (PMMA) remains challenging. Herein, we study the concept of applying QDs bearing polymer ligands matching the composition of the LSC matrix to reduce aggregation and the resulting parasitic waveguide losses caused by scattering. We report the synthesis and characterization of a thiol-terminated PMMA-based ligand and its successful ligand exchange reaction with copper indium disulfide/zinc sulfide (CIS/ZnS) QDs. QDs bearing PMMA ligands are then applied as down-converting phosphors embedded in a PMMA waveguide. The resulting QD-based LSCs were found to have lower optical scattering with higher loading as a result of reduced aggregation in the devices.

Luminescent solar concentrators (LSCs) use down-converting luminophores embedded in a waveguide to absorb sunlight and deliver high irradiance, narrowband output light for driving photovoltaic and other solar energy conversion devices. Achieving a technologically useful level of optical gain requires bright, broadly absorbing, large-Stokes-shift luminophores incorporated into low-loss waveguides, a combination that has long posed a challenge to the development of practical LSCs. The recent introduction of giant effective Stokes shift semiconductor nanocrystal (NC) phosphors for LSC applications has led to significant performance improvements by increasing solar absorption while reducing escape cone and nonradiative losses compounded by reabsorption, placing increased emphasis on the importance of minimizing parasitic waveguide losses caused by scattering from NC aggregates and optical imperfections. Here, we report a detailed analysis of optical losses in polymer−NC composite waveguide LSCs based on CuInS2/CdS NC phosphors, which have been shown to provide best-in-class performance in largearea, semitransparent concentrators. A comprehensive analytical optical model is introduced enabling quantification of parasitic waveguide, scattering, escape cone, and nonradiative relaxation losses on the basis of distance-dependent edge-emission measurements. By examining the effect of NC loading, we show that NC clustering in polymer composite waveguides leads to light scattering losses that ultimately limit efficiency at large geometric gain. By optimizing NC concentration, optical power efficiencies up to 5.7% under AM1.5 illumination are demonstrated for devices having a geometric gain G = 6.7×, with limiting achievable efficiencies predicted to exceed 10%. ■ INTRODUCTION Solar collectors capable of simultaneous spatial and spectral optical concentration can open new avenues for improving solar conversion efficiency, scalability, and cost by providing high brightness, narrow-bandwidth light optimized for photovoltaic (PV) or photochemical conversion. Luminescent solar concentrators (LSCs), which concentrate light in an inexpensive dielectric waveguide by active frequency shifting, have emerged as one of the most promising approaches for practical solar concentration (Figure 1A). LSCs employ downshifting luminophores to increase the spectral power of photons trapped in a waveguide by total internal reflection, theoretically enabling conversion efficiencies superior to other concentrator designs. PV cells or other converters coupled to the concentrator edges receive intense, bandgap-matched light, while excess energy from above-bandgap light is dissipated over the full area of the LSC. LSCs can concentrate diffuse as well as direct sunlight, are tolerant to partial shading, and can be semitransparent, making them particularly useful for buildingintegrated PV applications such as energy-harvesting window layers. Because some energy is sacrificed in the absorption, downshifting, and transport of sunlight by an LSC, achieving the required high levels of efficiency has long been an obstacle for practical concentrators. The theoretical performance limits of an LSC are established thermodynamically by the optical properties of the luminophores. The upper energy efficiency obeys the single-bandgap Shockley−Queisser limit, and the limiting energy concentration ratio (CRlim) depends on the Stokes shift. For visible to near-IR luminophores, thermodynamically limited theoretical energy concentrations of CRlim > 10 are predicted at room temperature for a Stokes shift of ∼500 meV, corresponding to >10 W m−2 of narrowband output. In practice, however, the performance of LSCs falls far below these thermodynamic limits, mainly because of four Received: December 8, 2016 Revised: January 19, 2017 Published: January 23, 2017 Article

Luminescent solar concentrators (LSCs) use down-converting luminophores embedded in a waveguide to absorb sunlight and deliver high irradiance, narrowband output light for driving photovoltaic and other solar energy conversion devices. Achieving a technologically useful level of optical gain requires bright, broadly absorbing, largeStokesshift luminophores incorporated into lowloss waveguides, a combination that has long posed a challenge to the development of practical LSCs. The recent introduction of giant effective Stokes shift semiconductor nanocrystal (NC) phosphors for LSC applications has led to significant performance improvements by increasing solar absorption while reducing escape cone and nonradiative losses compounded by reabsorption, placing increased emphasis on the importance of minimizing parasitic waveguide losses caused by scattering from NC aggregates and optical imperfections. Here, we report a detailed analysis of optical losses in polymerNC composite waveguide LSCs based on CuInS2/CdS NC phosphors, which have been shown to provide bestinclass performance in large area, semitransparent concentrators. A comprehensive analytical optical model is introduced enabling quantification of parasitic waveguide, scattering, escape cone, and nonradiative relaxation losses on the basis of distancedependent edgeemission measurements. By examining the effect of NC loading, we show that NC clustering in polymer composite waveguides leads to light scattering losses that ultimately limit efficiency at large geometric gain. By optimizing NC concentration, optical power efficiencies up to 5.7% under AM1.5 illumination are demonstrated for devices having a geometric gain G = 6.7X, with limiting achievable efficiencies predicted to exceed 10%.

The electronic and optoelectronic properties of molecular semiconductor thin films are directly linked to their extrinsic nanoscale structural characteristics such as domain size and spatial distributions. In films prepared by common solution-phase deposition techniques such as spin casting and solvent-based printing, morphology is governed by a complex interrelated set of thermodynamic and kinetic factors that classical models fail to adequately capture, leaving them unable to provide much insight, let alone predictive design guidance for tailoring films with specific nanostructural characteristics. Here we introduce a comprehensive treatment of solution-based film formation enabling quantitative prediction of domain formation rates, coverage, and spacing statistics based on a small number of experimentally measureable parameters. The model combines a mean-field rate equation treatment of monomer aggregation kinetics with classical nucleation theory and a supersaturation-dependent critical nucleus size to solve for the quasi-two-dimensional temporally and spatially varying monomer concentration, nucleation rate, and other properties. Excellent agreement is observed with measured nucleation densities and interdomain radial distribution functions in polycrystalline tetracene films. Numerical solutions lead to a set of general design rules enabling predictive morphological control in solution-processed molecular crystalline films.

The performance of colloidal CuInS2/CdS nanocrystals as phosphors for full-spectrum luminescent solar concentrators has been examined. Their combination of large solar absorption, high photoluminescence quantum yields, and only moderate reabsorption produces the highest projected flux gains of any nanocrystal luminophore to date.

Luminescent solar concentrators (LSCs) harvest sunlight over large areas and concentrate this energy onto photovoltaics or for other uses by transporting photons through macroscopic waveguides. Although attractive for lowering solar energy costs, LSCs remain severely limited by luminophore reabsorption losses. Here, we report a quantitative comparison of four types of nanocrystal (NC) phosphors recently proposed to minimize reabsorption in large-scale LSCs: two nanocrystal heterostructures and two doped nanocrystals. Experimental and numerical analyses both show that even the small core absorption of the leading NC heterostructures causes major reabsorption losses at relatively short transport lengths. Doped NCs outperform the heterostructures substantially in this critical property. A new LSC phosphor is introduced, nanocrystalline Cd(1-x)Cu(x)Se, that outperforms all other leading NCs by a significant margin in both small- and large-scale LSCs under full-spectrum conditions.

Optical concentration can lower the cost of solar energy conversion by reducing photovoltaic cell area and increasing photovoltaic efficiency. Luminescent solar concentrators offer an attractive approach to combined spectral and spatial concentration of both specular and diffuse light without tracking, but they have been plagued by luminophore self-absorption losses when employed on practical size scales. Here, we introduce doped semiconductor nanocrystals as a new class of phosphors for use in luminescent solar concentrators. In proof-of-concept experiments, visibly transparent, ultraviolet-selective luminescent solar concentrators have been prepared using colloidal Mn(2+)-doped ZnSe nanocrystals that show no luminescence reabsorption. Optical quantum efficiencies of 37% are measured, yielding a maximum projected energy concentration of ∼6× and flux gain for a-Si photovoltaics of 15.6 in the large-area limit, for the first time bounded not by luminophore self-absorption but by the transparency of the waveguide itself. Future directions in the use of colloidal doped nanocrystals as robust, processable spectrum-shifting phosphors for luminescent solar concentration on the large scales required for practical application of this technology are discussed.

ADVERTISEMENT RETURN TO ISSUEPREVCommunicationNEXTSterically Engineered Perylene Dyes for High Efficiency Oriented Fluorophore Luminescent Solar ConcentratorsWillie E. Benjamin†, Darren R. Veit†, Matt J. Perkins†, Edward Bain†, Kelsey Scharnhorst†, Stephen McDowall‡, David L. Patrick*†, and John D. Gilbertson*†View Author Information†Department of Chemistry and ‡Department of Mathematics, Western Washington University, Bellingham, Washington 98225, United States*E-mail: [email protected]*E-mail: [email protected]Cite this: Chem. Mater. 2014, 26, 3, 1291–1293Publication Date (Web):January 6, 2014Publication History Received6 October 2013Revised5 January 2014Published online23 January 2014Published inissue 11 February 2014https://doi.org/10.1021/cm403286vCopyright © 2014 American Chemical SocietyRIGHTS & PERMISSIONSACS Editors' ChoiceArticle Views3707Altmetric-Citations46LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (698 KB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information SUBJECTS:Absorption,Chromatography,Solubility,Imaging probes,Order Get e-Alerts

Luminescent waveguides (LWs) occur in a wide range of applications, from solar concentrators to doped fiber amplifiers. Here we report a comprehensive analysis of escape-cone losses in LWs, which are losses associated with internal rays making an angle less than the critical angle with a waveguide surface. For applications such as luminescent solar concentrators, escape-cone losses often dominate all others. A statistical treatment of escape-cone losses is given accounting for photoselection, photon polarization, and the Fresnel relations, and the model is used to analyze light absorption and propagation in waveguides with isotropic and orientationally aligned luminophores. The results are then compared to experimental measurements performed on a fluorescent dye-doped poly(methyl methacrylate) waveguide.

iv Acknowledgments v List of Figures ix List of Tables xii List of Abbreviations xiii Chapter 1: Introduction 1.0 Overview 1 1.1 Global Energy Consumption and the Need for Alternative Fuel Sources 1 1.2 Economics of Solar Power 3 1.3 Methods of Reaching $1/Wp 5 1.3.1 Increasing the Efficiency of Photovoltaic Cells 5 1.3.2 Using Cheaper Materials To Achieve $1/Wp 8 1.4 Methods of Solar Concentration 10 1.4.1 High Gain Solar Concentrators 11 1.4.2 Low Gain Solar Concentrators 11 1.5 Summary 12 Chapter 2: Luminescent Solar Concentrators 2.0 Overview 15 2.1 Description of LSCs 15

In the inverse stationary transport problem through anisotropic attenuating, scattering, and refractive media, the albedo operator stably determines the gauge equivalent class of the attenuation and scattering coefficients.

Three dimensional anisotropic attenuating and scattering media sharing the same albedo operator have been shown to be related via a gauge transformation. Such transformations define an equivalence relation. We show that the gauge equivalence is also valid in media with non-constant index of refraction, modeled by a Riemannian metric. The two dimensional model is also investigated.

For anisotropic attenuating media, the albedo operator determines the scattering and the attenuating coefficients up to a gauge transformation. We show that such a determination is stable.

We model the effects of dye molecule alignment on the collection efficiency of luminescent solar concentrators (LSCs). A Monte Carlo model for photon transport in LSC’s is derived and utilized, which incorporates the effects of fluorescent-dye-molecular alignment and the subsequent control over absorption, emission, and propagation properties. We focus on the effects of molecular alignment statistics on photon absorption and subsequent emission, including polarization and propagation direction imparted by dipole direction, to model device light-capture efficiency, defined as the ratio of the amount of light reaching particular slab edges to that incident on a face. We find that modest control of alignment, coupled with reasonable and attainable emission-absorption dipole angles, can produce very large collection efficiencies for a range of device parameters. We note that efficiencies for small values of dye molecule Stoke’s shift may be made as large as those for homogeneous (unaligned) systems with large Stoke’s shift.

Optical tomography is the use of near-infrared light to determine the optical absorption and scattering properties of a medium M ⊂ ℝn. If the refractive index is constant throughout the medium, the steady-state case is modeled by the stationary linear transport equation in terms of the Euclidean metric and photons which do not get absorbed or scatter travel along straight lines. In this expository article we consider the case of variable refractive index where the dynamics are modeled by writing the transport equation in terms of a Riemannian metric; in the absence of interaction, photons follow the geodesics of this metric. The dataone has is the measurement of the out-going flux of photons leaving the body at the boundary. This may be knowledge of both the locations and directions of the exiting photons (fully angularly resolved measurements) or some kind of average over direction (angularly averaged measurements). We discuss the results known for both types of measurements in all spatial dimensions.

In optical tomography one seeks to use near-infrared light to determine the optical absorption and scattering properties of a medium X ⊂ ℝ n . If the refractive index is constant throughout the medium, the steady-state case is modeled by the stationary linear transport equation in terms of the Euclidean metric. In this work we consider the case of variable refractive index where the dynamics are modeled by writing the transport equation in terms of a Riemannian metric; in the absence of interaction, photons follow the geodesics of this metric. In particular we study the problem where our measurements allow the application of an in-going flux depending on both position and direction, but we allow only a weighted average measurement of the out-going flux. We show that making measurements on all of ∂ X determines the extinction coefficient and that once this is known, under additional assumptions, measurements at a single point on ∂ X determine the scattering kernel.

ABSTRACT Optical tomography means the use of near-infrared light to determine the optical absorption and scattering properties of a medium. In the stationary Euclidean case the dynamics are modeled by the radiative transport equation, which assumes that, in the absence of interaction, particles follow straight lines. Here we shall study the problem in the presence of a Riemannian metric where particles follow the geodesic flow of the metric. In particular, we study the problem in dimension two, where the analysis is more delicate than in the higher dimensional cases.

The stationary linear transport equation models the scattering and absorption of a low-density beam of neutrons as it passes through a body.In Euclidean space, to a first approximation, particles travel in straight lines.Here we study the analogous transport equation for particles in an ambient field described by a Riemannian metric where, again to first approximation, particles follow geodesics of the metric.We consider the problem of determining the scattering and absorption coefficients from knowledge of the albedo operator on the boundary of the domain.Under certain restrictions, the albedo operator is shown to determine the geodesic ray transform of the absorption coefficient; for "simple" manifolds this transform is invertible and so the coefficient itself is determined.In dimensions 3 or greater, we show that one may then obtain the collision (or scattering) kernel.